Operational issues in isothermal calorimetry Lars Wadsö ⁎ Building Materials, Lund University, Box 118, 221 00 Lund, Sweden abstract article info Article history: Received 21 September 2009 Accepted 18 March 2010 Keywords: Calorimetry Calibration Baselines Time constants References Isothermal (heat conduction) calorimetry is a general technique to study processes through the thermal power they produce. This paper deals with operational issues concerning isothermal calorimeters. In this paper it is shown that steady-state and pulse calibrations give the same result; that the use of mobile heaters (placed in the reaction ampoule) give more accurate results than fixed heaters (placed in the ampoule holder); and that at least the tested calorimeter had calibration coefficients that were independent of the thermal power level. It is shown that well balanced references are necessary to get low noise and low drifts. It is discussed how baselines should be measured. The influence of temperature and sample size is also discussed and it is shown that large cement paste samples with high thermal powers will show an accelerated reaction. Finally, the thermal dynamics of a heat conduction calorimeter is discussed. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Isothermal (heat conduction) calorimetry is a laboratory method to measure thermal power (heat production rate, heat rate) as a function of time on small samples at constant temperature. It has found uses in many areas of science and technology, for example in pharmaceutics [1], microbiology [2] and cement science [3]. In an isothermal (heat conduction) calorimeter the sample (typically of 1- 10 g) is in an ampoule that is inserted into an ampoule holder in contact with a heat flow sensor on a thermostated heat sink. As heat – endothermal or exothermal – is produced in the sample, the sample temperature will change, and this gives rise to a heat flow that is measured by the heat flow sensor as a voltage. The heat flow sensor work by the Seebeck principle in which a temperature difference over the sensor produces a voltage. All heat conduction calorimeters are twin calorimeters, i.e., there is also a reference system in which an inert sample is placed, and it is the difference between the output of the sample and the reference sensors that is recorded. This output can be converted to thermal power as a function of time and integrated to get heat. The thermal power is related to the rate of the studied process, while the produced heat is a function of the extent of the reaction [4,5]. There are in practice three properties of a calorimeter that should be considered during evaluations of calorimetric results: the calibration coefficient ε (W V -1 ), the baseline U 0 (V) and the time constant τ (s). The use of these can be explained with reference to the following two equations for the calculation of thermal power P (W) from the voltage signal U (V): P = εðU-U 0 Þ ð1Þ P c = P + τ dP dt ð2Þ Eq. (1) is the standard equation to use to calculate thermal power from voltage; Eq. (2) is the Tian equation (named after a French scientist who developed isothermal calorimeters in the 1920s) to correct rapid processes for the time lag of an instrument (P c is the corrected thermal power). Throughout this paper the term “thermal power” will be used when referring to the rate of heat production in the sample. It is also common to see the terms “heat flow” and “heat flow rate”, but these terms are best reserved for discussions on heat flows within calorimeters. Note that not all the above mentioned three properties are needed for all types of measurements. For the measurement of the heat produced during 7 days of cement paste hydration both the calibration coefficient and the baseline are important; for the study of the rapid initial reactions that take place when cement is mixed with water the Tian correction – and thus also the time constant – is needed in order to separate different events; and for studies of the retardation of the cement hydrations by an admixture (additive), none of these three parameters are actually needed as it is only, e.g., the time of the peak of the main hydration that is of interest. Although isothermal calorimeters are used in many cement laboratories, there is an uncertainty on how accurate these instruments are. This has delayed the standardization of isothermal calorimetry in the cement field, where many companies make extensive use of isothermal calorimetry within their organizations, while still relying on traditional and standardized calorimetric techniques in communicating Cement and Concrete Research 40 (2010) 1129–1137 ⁎ Tel.: +46 705 596989; fax: +46 46 2224427. E-mail address: lars.wadso@byggtek.lth.se. 0008-8846/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2010.03.017 Contents lists available at ScienceDirect Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp